The present disclosure relates, in general, to the field of solar power generation. More particularly, the present disclosure is directed to the protection or inhibition of freezing or solidification of a heat transfer fluid in a solar receiver.
A solar receiver is a primary component of a solar energy generation system whereby sunlight is used as a heat source for the eventual production of superheated high quality steam that is used to turn a turbine generator, and ultimately produce electricity using the Rankine cycle or provide steam for other thermal processes.
Generally, the solar receiver is positioned on top of an elevated support tower which rises above a ground level or grade. The solar receiver is strategically positioned in an array of reflective surfaces, such as a field of heliostats (or mirrors), that collect rays of sunlight and redirect or reflect those rays to the heat absorbing surfaces of the solar receiver. This solar energy is then absorbed by the working heat transfer fluid (HTF) flowing through the solar receiver. The reflective surfaces may be oriented in different positions through the day to track the sun and maximize reflected sunlight to the heat absorbing surfaces.
The solar receiver is an assembly of tubes with water, steam, molten salts, or other heat transfer fluid (HTF) flowing inside the tubes. The HTF inside the tubes of the receiver absorbs the concentrated solar energy, causing the HTF to increase in temperature and/or change phases, so that the HTF captures the solar energy. The heated HTF is then either directly routed to a turbine generator to generate electrical power or is indirectly routed to a storage tank for later use.
A common problem in solar receivers relates to temperature drops that occur during periods of solar inactivity, e.g., dense/continuous cloud cover, nightfall, and the like. During periods where solar activity, e.g., heat, is noticeably absent, the temperature of the heat transfer fluid may drop to temperatures that approach or fall below freezing/solidification. Such periods can occur in varying climates, including desert environments, where concentrated solar power (CSP) plants are primarily located. When using water as a heat transfer fluid, freezing of the fluid within the receiver may occur as external temperatures drop (e.g. winter) and offsetting heat from the sun is unavailable. The evaporator tube panels, which are filled with water, are exposed to ambient conditions and are particularly in danger of freezing. If the receiver is not drained, the heat transfer fluid will expand and could rupture the tube(s).
Current preventive measures include draining the heat transfer fluid from the solar receiver so as to prevent damage to components of the solar receiver caused by freezing. These measures are not practical for commercial plants. There are also other disadvantages, including wasting the drained heat transfer fluid and chemicals, consumption of nitrogen (to displace air for corrosion control) which increases operating costs, the time needed to refill the solar receiver and the resulting increase in startup time (and decreased availability), and the discarding of thermal energy in the heat transfer fluid contained in the steam drum or vertical separator (discussed later). In addition, the receiver is at risk for scaling and corrosion during the time period required to get the water quality back to the proper chemistry. For these reasons, it would be advantageous to leave the solar receiver full of HTF.
Electrical trace heating is a system used to maintain or raise the temperature of some instrument tubing and small bore piping. Generally, an electrical heating element is run in thermal contact along the length of a pipe, and the pipe is then covered with thermal insulation to retain heat losses from the pipe. However, it is not practical or cost-effective to heat trace all of the water-filled solar receiver panel tubes due to their large quantity and small size. For example, there could be several hundred evaporator tubes in a solar receiver. Additionally, there is also a need for a control system for monitoring and activating a freeze/solidification protection system to reduce or prevent damage and enhance the efficiency of a solar receiver.